Process for drying water-wet hmx with lower alkanols



United States Patent 3,386,993 PROCESS FOR DRYING WATER-WET HMX WHTHLOWER ALKANOLS Madhav R. Bhagwat, Indian Head, Md., assignor to theUnited States of America as represented by the Secretary of the Navy N0Drawing. Filed Mar. 18, 1965, Ser. No. 440,958 4 Claims. (Cl. 260239)ABSTRACT OF THE DISCLOSURE A process for removing water from water-wetHMX which includes forming a homogeneous mixture of waterwet HMX with alower alkanol solvent of the type in which HMX is relatively insolubleand water is infinitely soluble, and thereafter filter separating freeflowing HMX.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

This invention relates generally to a procedure for processing wet HMX(cyolotetramethylenetetranitramine). More specifically, this inventionis directed to a new and improved method for drying water wet HMX.

HMX is shipped wet with water and isopropyl alcohol antifreeze in muslinbags and before it can be used in the production of solid propellants itmust be dried to a water content that does not exceed about 5%. In themethod presently employed the initial separation is effected by suctionfiltration. The HMX recovered from the filter has the appearance of awhite plaster, the consistency of cold cream, and a water content thatvaries from about 25 to 40%. This consistency prevents furtherseparation by filtration and the drying operation is completed in a dryhouse at about 160 F.

A disadvantage of this drying method is that it does not produce auniform product. The outer layer dries faster than the inner parts andthis results in a dry HMX, that is lumpy and uneven in its moisturecontent and physical makeup. Before being used in the propellant thehard chunks in the dry material must be manually broken into smallpieces and softened by a soaking in acetone for twenty-four hours. Inspite of this soaking, fine hard agglomerates are left which cause quitea bit of difliculty during production of the propellant.

Another disadvantage is that the drying operation is too time consumingin that it takes anywhere from three to five days to dry the wet HMX.

A further disadvantage is that this process requires too much manualhandling of the friction sensitive HMX and thus exposes personnel to thehazards of an explosion.

Still another disadvantage is that drying large amounts of HMX atelevated temperatures (160 F.) creates a potential safety hazard.

Accordingly, it is an object of this invention toproduce dry HMX by aprocess which does not suffer from the aforementioned disadvantages.

It is a further object of this invention to dry HMX by a process whichproduces a uniform product.

Another object is to dry HMX by a process which is safe, economical, andmore efficient.

The objects of this invention are accomplished by replacing the water inthe wet HMX with a solvent in which HMX is relatively insoluble andwater is infinitely soluble. It is preferred that the solvent becompatible with the propellant composition so that no further processingis necessary. As examples of solvents that are operable in thisinvention there may be mentioned: the lower alkanols such as methanol,ethanol and isopropanol. Of these ethanol is preferred since it isrelatively inexpensive. It is to be understood, however, that theinvention is not to be limited to the abovementioned solvents.

This process is based upon the unexpected finding that although HMXparticles that are coated with water form a lumpy plaster-like mass thatis very difficult to filter, HMX particles coated with a solvent meetingthe above requirements forms a suspension that is easily filtered. Thus,it is essential to this invention that there be a uniform and intimatemixing of the solvent with the water wet HMX so that the water coatingis replaced with solvent.

More particularly, the process of this invention comprises the followingsteps:

Step 1. Draining the wet HMX bags to remove excess water and isopropylalcohol. This reduces the water content to an amount that is not inexcess of about 40% and is usually in the range of between 25 to 40%.

Step 2.Opening the bags and feeding the wet HMX into a premix unit inwhich the HMX is intimately mixed with the solvent to obtain ahomogeneous dispersion.

Step 3.Diluting the premix (Step 2) with additional solvent.

Step 4.Filtering the free flowing suspension (Step 3) on a filter suchas a rotary vacuum filter.

Step 5.-Air drying the HMX filter cake from Step 4, if necessary, tocontrol the water and solvent content as required in the propellantformulations.

Step 6.-Recovering the solvent from Step 4 and recycling it to thesystem (Step 2 or 3).

The product from Step 5 can be directly slurried into acetone for use inthe propellant rnix.

Step 2 is the most crucial part of the process since a simple mixing ofthe solvent and water wet HMX will not produce a homogeneous dispersionbut will result in either a wrapping of the plaster-like HMX around theshaft of the stirrer or in the breaking of the wet mass intoagglomerates. It is only an intimate and uniform mixing that replaceswater coated on the HMX with sol vent and enables an easily filteredsuspension to be formed. This intimate mixing is accomplished inkneading type of equipment such as a horizontal or vertical type ofdough mixer but any kind of mixer that will cause complete homogenizingis operable. The amount of solvent added in the premix step is criticalsince the formation of the homogeneous mixture is dependent upon theremoval of water coating from the HMX particles. Thus the amount ofsolvent present should be sufficient to remove this water coating andthis amount usually ranges from about 0.25 to about 2 parts by weight ofsolvent (hereinafter all parts are by weight) per part of dry HMX withabout .5 to about 1 part of solvent per part of dry HMX being preferred.It is to be understood, however, that more than the critical amount ofsolvent could be added in the premix step. The amount of sol vent addedis only critical in the initial part of the mixing operation, i.e., thesolvent must be present within the above ranges only until a homogeneousmixture is formed and once formed any amount of solvent may be added.The solvent is usually added slowly so as to insure a thorough mixing ofthe solvent and water wet HMX. The speed of the mixer is not critical,but it is obvious to one skilled in the art that a slower rate of mixinggives a better dispersion.

Since once the homogeneous premix is formed the HMX can easily befiltered, there is nothing critical about the solvent addition or mixingin the dilution step. It is preferred, however, that there be some typeof agitation in this step in order to increase the contact between theHMX and solvent but the intimate mixing employed in the previous step isnot required. In this step enough solvent is added so that the totalamount of solvent is sufficient to either remove all the water that ispresent or remove sufficient water to bring the HMX tothe desiredmoisture content. Since in a filtering operation a more dilute solutiongives a faster filtration rate, this factor will also be taken intoaccount in determining the amount of solvent to be added. The amount ofsolvent that is added is usually sufficient to bring the total amount ofsolvent in the mixture to about 0.5 to 4 parts of total solvent per partof dry HMX with about one to two parts of total solvent per part of dryHMX being preferred. If the amount of solvent present in the premixoperation is sutficient to remove all of the water, this additional stepis not necessary. However, since a two-step opera.- tion usuallyproduces a better dispersion, this type of operation is preferred, withhalf of the total amount of solvent that is to be used being added inthe premix step.

This process is usually run at ambient temperatures and thus reduces thehazards of handling the sensitive HMX at elevated temperatures. Theprocess is entirely a mechanical operation and consists of a set of wellregulated chemical engineering unit operations that may be adapted to abatch or continuous process. The process is illustrated by not limitedby the following specific examples since various additionalmodifications can be made without deviating from the scope of theinvention.

EXAMPLE I A one quart stainless steel beaker fitted with a bafiie wasprovided with an air driven shaft having a propeller with S-shapedblades that gave a kneading action. 250 grams of wet HMX (30% water) wasslowly kneaded with 250 cc. of ethanol. In about eight minutes ahomogeneous suspension of HMX in alcohol was formed. An additional 250cc. of alcohol was slowly added. The diluted suspension was stirred forseven minutes and filtered on a Buchner funnel (filter paper) withpartial suction. Filtration time was nine minutes. The alcohol wet HMXfilter cake was about one-half inch thick. The moisture content of thecake, as determined by the Karl Fischer method, on three samples wasbetween 1.6% and 1.3%. Total volatiles in the cake (determined on a gramsample heated at 70 C. for one hour) was 16%. Final dry HMX was a freeflowing fine powder.

EXAMPLE II In this experiment the bench scale procedure was scaled up toproduce about 85 pounds of dry (2.4% water) HMX.

One hundred and thirty-five pounds of wet HMX (30% water) was slowlykneaded in a vertical dough mixer into 96 pounds of ethyl alcohol tosecure a free flowing premix suspension of HMX particles. The premix wasdiluted and stirred with 145 pounds of additional alcohol. The alcoholwas added gradually to maintain a uniform suspension. The suspension wasstirred for about twenty minutes and filtered. A stainless steel Nutschefilter of six feet diameter (muslin cloth) with steam jet suction wasused. 95.3 pounds of alcohol wet cake of thickness varying from A" to/2" was produced. Total volatiles in the filter cake-l3%, water contentof cake-2.4%.

Using the procedure of Example II between 900 to 1000 pounds of dry HMX,about 2 to 3% water was prepared. This HMX gave satisfactory results inpropellant formulations.

EXAMPLE III Equipment similar to Example I was used. 500 cc. of alcoholwas placed in the beaker and 1200 grams of wet HMX (20% water) wasslowly added while the stirrer was on. In addition 700 cc. of alcoholwas added and the kneading action was continued until a homogeneoussystem was formed.

The premix was transferred to another beaker that has no baflles and anordinary stirrer. The premix was slowly diluted with an additional 1200cc. of alcohol while stirring. The stirring was continued for thirtyminutes and then the mixture was filtered on a 10" diameter Buchner(filter paper) with about 30 (Hg) vacuum. The filtration was completedin fifteen minutes with the filter cake having an average thickness ofof an inch. The HMX contained two to three percent water.

EXAMPLE IV Example III was repeated but in the filtration coarse muslinwas used instead of filter paper. There was very littie change in thefiltration. The cake contained 1.9% water.

EXAMPLE V An operation similar to Example III was employed on 1200 gramsof wet HMX (23.7% water). In this operation 1050 cc. of ethanol (95%)was used in the premix operation and 1950 cc. of ethanol in the dilutionstep.

The total volatiles in the filter cake was 19.1% and the water contentwas 2%.

Net weight of alcohol recovered from the filtrate was 1950.4 grams.

EXAMPLE VI 600 grams of wet HMX (23.7%) was added to cc. of ethanol(95%) in a vertical dough type of mixer. An additional 750 cc. ofethanol was added and the mixing was continued for twenty minutes. Ahomogeneous mixture was formed. The diluent step was carried out in thesame mixer with an increased speed of agitation. 750 cc. of ethanol wasslowly added and the agitation continued for fifteen minutes. Themixture was filtered similarly to Example 111. The total volatiles inthe cake was 22.4% and the water content 2.8%.

The process of this invention has the following major advantages:

1. It can be set up as well controlled chemical engineering unitoperations, thus producing a uniform prodnet.

(2) It does not affect the physical properties of the HMX.

(3) It can be run at ambient temperatures and thus it eliminates thehazards involved in handling HMX at ele vated temperatures.

(4) It reduces the hazards of an explosion caused by friction since theHMX is handled wet with either water or solvent throughout theoperation.

(5) It is more efficient since the overall time for drying the HMX isless than in prior processes.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A process for drying water wet HMX which comprises,

slowly adding from about 0.25 to 2 parts of a solvent per part of dryHMX to the water wet HMX, said solvent being a lower alkanol solvent inwhich HMX is relatively insoluble and Water is infinitely soluble,kneading the solvent and HMX during the addition, said kneading beingeffected until a homogeneous mixture is formed, diluting the homogeneousmix- 5 6 ture with additional solvent, and filter separating theReferences Cited HMX from the mixture. 2. The process of claim 1 whereinthe water wet HMX UNITED STATES PATENTS contains initially from about 25to 40% water. 1,398,911 11/1921 5661 260-223 3. The process of claim 2in which the additional sol- 5 2,102,799 12/ 1937 l y et 6 3 vent isadded in an amount such that the total solvent 3,297,631 1/ 1967 Wrightet 260239 present is in an amount from about 0.5 to 4 parts per part ofdry HMX. FOREIGN PATENTS 4. The process of claim 3 in which the initialsolvent 590,851 1/1960 Canada.

is present in an amount from about 0.5 to 1 part per 10 part of dry HMX.ALTON D. ROLLINS, Primary Examiner.

